The present invention relates to a semiconductor device and a process for producing the same. More particularly, the present invention relates to an improvement of a high electron mobility transistor, hereinafter referred to as a HEMT, and an improved process for producing a HEMT.
The HEMT is one of the active semiconductor devices and its operating principle is completely distinguishable from that of the silicon ICs and the GaAs FETICs.
The HEMT has a first single crystalline semiconductor layer, a second single crystalline semiconductor layer having an electron affinity different from that of the first single crystalline semiconductor layer and forming a heterojunction with respectto the first single crystalline semiconductor layer, and a gate electrode on the second single crystalline semiconductor layer. An electron-storing layer is formed in proximity to the heterojunction due to the difference in the electron affinity and is used as the conduction channel of the HEMT. The electron mobility of the HEMT is approximately 10 times that of the GaAs FETICs and approximately 100 times that of the silicon ICs, at the temperature of liquid nitrogen (77K). The electron-storing layer mentioned above contains a quasi two-dimensional electron gas, the electrons of which gas being the predominant current-conduction carriers of the HEMT. In addition, the sheet-electron concentration of the quasi two-dimensional electron gas is controlled by applying voltage through the gate electrode. The impedance of the conduction channel is controlled by a pair of electrodes located beside the gate electrode. A very high electron mobility of the HEMT at a low temperature, for example at 77K, is due to the fact that the electron mobility of the quasi two-dimensional electron gas is very high at such a temperature where impurity scattering predominantly controls the electron mobility. The quasi two-dimensional electron gas is formed in proximity to the heterojunction, as stated above, and the thickness of the quasi two-dimensional electron gas is approximately the spreading amount of electron waves or on the order of ten angstroms. The relationship between the quasi two-dimensional electron gas and one of the single crystalline semiconductor layers which supplies the electrons into the electron-storing layer is a spatially separated one, with the result that the electron mobility of the quasi two-dimensional electron gas is not lessened due to the reduction of impurities in one of the single crystalline semiconductor layers. As a result, it is possible to achieve a very high electron mobility at such a low temperature where the electron mobility is predominantly controlled by impurity scattering.
The background of the present invention is explained more in detail with reference to FIG. 1.
Referring to FIG. 1, Ec.sub.1 indicates the energy of the electrons in the second single crystalline semiconductor layer having a low electron affinity, for example, an N-doped AlGaAs (aluminum gallium arsenide) layer, and Ec.sub.2 indicates the energy of the electrons in the first single crystalline semiconductor layer having a high electron affinity, for example, an undoped GaAs layer. Because of the difference in the electron affinity, a substantial part of the electrons in the N-doped AlGaAs layer are attracted to the undoped GaAs layer and quasi two-dimensional electron gas (2DEG) is formed as shown in FIG. 1 while the positively-ionized impurities (+) remain in the N-doped AlGaAs layer. The symbol "E.sub.F " indicates the Fermi level, and the symbol "Ecg" indicates an energy gap, at the interface of the heterojunction, between the N-doped AlGaAs layer and the undoped GaAs layer. Two semiconductors capable of forming the heterojunction of the HEMT should have a sufficient difference in electron affinity, as was explained above. Furthermore, since the region near the interface of the heterojunction should be free of any crystal defects, the lattice constants of the two semiconductors should be close to one another. In addition, in order to attain a satisfactorily high energy gap (Ecg) at the interface of the heterojunction, the difference between the energy gap in the two semiconductors should be great. The types of semiconductors which satisfy the above-described three properties, i.e. electron affinity, lattice constant, and energy gap, are numerous, as shown in Table 1.
TABLE 1 ______________________________________ Band Lattice Electron Gap Constants Affinity No. Semiconductors (eV) (.ANG.) (eV) ______________________________________ 1 Aluminum gallium 1.8 5.657 3.77 arsenide (Al.sub.0.3 Ga.sub.0.7 As) Gallium arsenide 1.43 5.654 4.07 2 Aluminum gallium 1.8 5.657 3.77 arsenide (Al.sub.0.3 Ga.sub.0.7 As) Germanium 0.66 5.658 4.13 3 Gallium arsenide 1.43 5.654 4.07 Germanium 0.66 5.658 4.13 4 Cadmium telluride 1.44 6.477 4.28 Indium antimonide 0.17 6.479 4.59 5 Gallium Antimonide 0.68 6.095 4.06 Indium arsenide 0.36 6.058 4.9 ______________________________________
There are two types of operation of the HEMT, namely the normally on-type (depletion type) of operation and the normally off-type (enhancement type) of operation. The layer structure and the thickness of the first and second single crystalline semiconductor layers determine which type of operation the HEMT has. More specifically, in a case where the electron affinity of the second single crystalline semiconductor layer is greater than that of the first single crystalline semiconductor layer, the type of operation is the normally on-type when the metallurgical thickness of the second single crystalline semiconductor layer (upper layer) is greater than a certain critical amount. This certain critical amount depends on the aforementioned three parameters and on the properties of the gate which is to be formed on the upper layer. The type of operation is the normally off-type when the metallurgical thickness of the upper layer is smaller than the certain critical amount mentioned above. Similarly, in a case where the second (upper) and first (lower) single crystalline layers have a small and great electron affinity, respectively, the type of operation is the normally on-type when the metallurgical thickness of the lower layer is greater than a certain critical amount, which depends on the layer parameters. The type of operation is the normally off-type when the metallurgical thickness of the lower layer is smaller than the certain critical amount. The metallurgical thickness is hereinafter simply referred to as the thickness.
In the known HEMT, so that the input and/or output electrode can be electrically connected to the conduction channel with a low resistivity, one or both of the first and second single crystalline semiconductor layers are subjected to thermal diffusion so as to dope the layers with impurities at a high concentration to decrease the resistivity. Alternatively, one of or both of the first and second single crystalline layers are subjected to ion implantation followed by annealing of the implanted impurities for activation. Thermal diffusion and annealing after ion implantation are, however, undesirable with respect to the interface properties of one of the single crystalline semiconductor layers in which the electron-storing layer is to be formed. Especially when the heat treatment is carried out at such a temperature that the impurities contained in the other single crystalline semiconductor layer are liable to diffuse into one of the single crystalline semiconductor layers. The electron mobility in the electron-storing layer is lessened when the impurities are diffused in the electron-storing layer. Therefore, the impurities contained in one of the two single crystalline semiconductor layers of the HEMT having different electron affinities should be strictly spatially separated from the other layer, and judging from the fact that the quasi two-dimensional electron gas has a width of electron waves, the diffusion of a few atom layers of the impurities is detrimental to the high electron mobility of the conduction electrons. Without such a low resistivity region having a high impurity concentration between the electrode and the conduction channel composed of the quasi two-dimensional electron gas, the parasitic resistance between the electrode and the conduction channel cannot be lowered sufficiently even through an alloying of the electrode metal.
By such a low resistivity region, not only is ohmic contact between the electrode and one of these single crystalline semiconductor layers in which the electron-storing layer is formed, realized, but also an electrical connection between an electrode and the other single crystalline semiconductor layer is formed which supplies the electrons to the electron-storing layer. In every type of HEMT, the predominant conduction carriers should be the electrons in the quasi two-dimensional electron gas in the electron-storing layer, and, therefore, an output electrode of the HEMT must be electrically connected to the electron-storing layer. It is, however, unnecessary for an output electrode with a low resistivity to be electrically connected to one of the single crystalline semiconductor layers which supplies the electrons to the electron-storing layer. The electrical connection between an output electrode and one single crystalline semiconductor layer is rather harmful because the electrons in this layer have a low electron mobility which can generate current and thus decrease the operation speed of the HEMT. In the N-doped Al.sub.0.3 Ga.sub.0.7 As layer, which is used as one of the single crystalline semiconductor layers for supplying the electrons to the electron-storing layer, the silicon atoms, i.e. the N-type conductivity impurities, do not have, at 77K, an energy exceeding the energy gap of the conduction band of the Al.sub.0.3 Ga.sub.0.7 As, with the result that the freezing of carriers occurs at 77K. The temperature at which the freezing of carriers occurs depends on the main composition of one single crystalline semiconductor layer and the kind of N-type conductivity impurities. When the HEMT comprising the N-doped Al.sub.0.3 G.sub.0.7 As is to be operated at room temperature, the freezing of carriers does not occur but the free electrons generated from the silicon atoms can be one type of conduction carriers. Since the HEMT can be used at room temperature and exhibits an operation speed twice as high as that of conventional silicon ICs, it is important to prevent the electrons of the silicon atoms from participating in the generation of current.